1. Field of Invention
This invention is directed to a method and apparatus for continuously combining two, or more, distinct liquids, while maintaining substantially fixed relative proportions, over a range of flow rates.
2. Description of Prior Art
The scope of field of this invention includes applications as diverse as medical treatment, manufacturing processes, and painting. A primary application of this invention is painting with high-performance two-part paints; an application that well illustrates the advantages and limitations of the prior art for combining liquids in specific proportions.
Such two-part paints typically consist of a pigment carrier as one component and a hardener as another component. These paints must be mixed in the proper proportions, at the time of, or shortly before, use. Prior art has solved the proportional combining problem associated with these paints in two effective ways.
One approach, most appropriately used when the required amounts of paint are small, is to premeasure the liquid components and package them in separate compartments that are joined, but have an impenetrable barrier between them. The barrier must be removed just prior to use, and the components combined and mixed to provide a paint that is suitable for immediate use. For example, U.S. Pat. Nos. 5,405,051 and 5,431,303 to Miskell (1995) disclose a dispenser wherein the reactive components are held in two separate containers and are maintained separate from one another by a heat-fusible plug in the first case and by a pierceable membrane in the second case. Removing the barrier permits the reactants to combine and mix.
Another example of this approach is found in U.S. Pat. No. 4,988,017 to Schrader, et. al. (1991) which discloses an aerosol container system with two separated chambers, containing liquids to be mixed just prior to use.
The advantages of using such premeasured and prepackaged products include:
(a) Eliminating the need for bulk handling and pouring of potentially toxic chemicals. PA1 (b) The ability to purchase and store only small amounts of products with resulting economic and safety benefits. PA1 (c) The ability to dispose of the package after use, greatly facilitating cleanup, and significantly reducing the use of cleaning solvents, with resulting environmental benefits. PA1 (d) Light weight and small size, making a portable system practical. PA1 (a) Once the components are mixed, they must be used quickly, or they become useless. Similarly, any paint left over after job completion, being already mixed, is rendered useless. Since high-performance paints of the described type are expensive, this is seen to be a significant disadvantage. PA1 (b) Since all of the liquids are mixed at the same time, the curing time begins immediately, thus the last of the paint to enter the spray nozzle has been reacting for the entire time of painting. If the painting is interrupted, or cleaning of the spray apparatus delayed slightly, cleaning can become unreasonably difficult or even impractical. PA1 (c) Joined component packages preclude the ability to selectively combine components to achieve desirable characteristics, such as, for example, extra fast drying or hardening. PA1 (a) Big jobs can be handled more conveniently than with pre measured and prepackaged components. PA1 (b) Relative proportions of liquid components can be changed at will. PA1 (a) The expense of servo controlled metering, using controllers, transducers, etc. is very high for one work station. PA1 (b) Such systems, being relatively bulky, heavy, and delicate, are not easily portable, and for practical purposes require fixed location usage. PA1 (c) The amount of clean up required after use is significantly greater than that associated with prepackaged products. Correspondingly, the amount of cleaning solvent required is greater, with attendant cost and environmental penalties. PA1 (d) The sensitivity of servo system components, and the harsh environment of industrial painting operations necessitate regular maintenance and calibration by skilled technicians. PA1 (e) The use of electrically powered systems around solvent based painting operations requires extraordinary safety precautions. PA1 (a) To provide for the use of prepackaged liquid components as desired, with all attendant advantages, as presented above; while restricting combining to those component portions being instantaneously used. Thus the time allowable for job completion is many times greater than with previous prepackaged component systems. Additionally, any remaining liquids can be stored for future use. PA1 (b) To provide for the use of prepackaged liquid components, while supplying all downstream flow paths, as interior to a spray gun, for example, with an unreacted solvent-bearing mixture that continuously cleans rather than clogs. PA1 (c) To provide for the use of prepackaged liquid components, while preserving the ability to selectively combine components to achieve desired characteristics, and while maintaining the ability to change the relative proportions as required by the chosen components. PA1 (d) To provide for the use of prepackaged, selectively chosen cleaning solvents for in-place cleaning of fully assembled spray apparatus or applicators. PA1 (e) To provide an on-demand, fixed-proportion, continuous blending system at very low cost, relative to systems employing measuring, metering, and control components. PA1 (f) To provide a small, light-weight, and rugged liquid blending system that is easily portable and transportable. PA1 (g) To provide a fixed-proportion continuous blending system requiring substantially less cleanup effort and cleaning solvent than do servo controlled systems. PA1 (h) To provide a rugged and accurate blending system with disposable critical components, with no need for sophisticated maintenance and calibration. PA1 (i) To provide a non-electric, demand type, liquid blending system, suitable for use in flammable or explosive environments.
This art, while workable, has the following severe disadvantages:
A second approach described in prior art is to continuously combine two or more components into a continuous stream, while maintaining fixed relative proportions. U.S. Pat. No. 4,440,314 to Vetter et al. (1984) and U.S. Pat. No. 4,176,672 to Borberg (1979) teach this approach through the use of controllers, flow transducers, and servo controlled pumps. A similar approach is taught by U.S. Pat. No. 4,019,653 to Scherer, et. al. (1977) which teaches actuated valves instead of servo controlled pumps to regulate component flow. The advantages of the approach taught in these patents are:
The following significant disadvantages are associated with this prior art.